1. Field of the Invention
This invention relates to a water-based coating composition for elastomers which is suitable as a friction-reducing coating on elastomeric moldings. More particularly, the invention relates to water-based polyurethane compositions which establish a low coefficient of friction after application to typical elastomeric sealing profiles.
2. Description of the Related Art
There are a number of industrial applications which require elastomers that have to meet a number of requirements. This applies in particular to the sealing of window channels for moving glass parts and glass guide rails for glass windows in vehicles, especially motor vehicles. The elastomers are expected to show high heat resistance and weathering resistance under various climatic conditions. The surface of the elastomers is expected to have a very low friction coefficient, particularly for holding and guiding moving glass windows. The surfaces of the elastomer profiles on which the moving glass slides are normally coated with a film which has improved surface slip towards glass compared with the uncoated elastomer. There are various known processes for improving the surface slip of this coating.
DE-A-3335150 describes a polyurethane coating with beads or other particles embedded in the binder, the beads or other particles being larger than the layer thickness of the binder. These so-called xe2x80x9cspacersxe2x80x9d may be particles of glass, aluminium, polypropylene or polyvinyl chloride. A basic disadvantage of this procedure is that the peak and trough structure of the spacer-containing paint does not provide for optimal sealing so that unwelcome noise-creating drafts are able to penetrate. Another disadvantage is that the surface of the elastomer profiles thus coated is neither smooth nor glossy to the eye, but instead looks like emery cloth.
Another process which is described in U.S. Pat. No. 4,572,872 and U.S. Pat. No. 4,572,871 comprises adding silicones or fluororesins to a friction-reducing paint. Paints such as these have the disadvantage that, under extreme weather conditions, such as heat cold, wind and rain, to which the elastomer profile is normally exposed, the added silicones and fluororesins are depleted which results in a reduction in the surface slip of the elastomer profile towards glass and is therefore unwelcome. Another disadvantage is that the silicones are oils which are known to act as release agents. By reason of their characteristic properties, such release agents are dispersed relatively easily and can lead to difficulties, for example at the assembly stage, such as in the bonding and painting of bodywork parts. Accordingly, the use of silicone oils which have not been firmly bound is not welcomed by car manufacturers.
DE-A-3503479 proposes a coating composition containing a polyurethane prepolymer obtained by reacting a hydroxyl-terminated polyester polyol with an organic diisocyanate in a molar NCO:OH ratio of 0.6:1 to 0.95:1, a castor oil polyol and an isocyanate-terminated urethane prepolymer as a friction-reducing film. This coating composition is said also to contain a fluorocarbon resin, a liquid silicone oil and a small quantity of fillers and/or pigments. The coating composition is used in the form of a solution of the binder components in an organic solvent. There is no indication as to whether this binder system can be made up as a single-component system or multicomponent system for the user.
EP-A-655488 describes a urethane-resin-based coating composition for reducing friction which comprises a polyurethane paint containing a polyol and an isocyanate hardener and a polymeric powder of 11-nylon and/or 12-nylon with a melting point below the vulcanization temperature and a solubility parameter greater by at least 0.5 than the solubility parameter of the urethane paint. The polymeric powder is said to have a mean diameter of 5 to 500 xcexcm. Again, there is no indication as to whether the coating compositions in question can be formulated as storage-stable one-component systems.
U.S. Pat. No. 5,558,741 describes a process for the production of a window sealing profile in which the elastomeric sealing profile is extruded and the surface of the elastomeric sealing profile is then exposed for 0.5 to 60 seconds to a UV radiation of about 0.2 to 40 joules. The surface thus treated is then coated with a friction-reducing coating consisting of a resin binder and a lubricating pigment. Fluorocarbon polymers, such as PTFE, and silicone lubricants are mentioned as preferred lubricants. The binder is not described in detail, but is merely said to be obtainable from Acheson Industries Inc. under the name of xe2x80x9cEMRALONxe2x80x9d.
DE-A-19608057 describes a water-dilutable sealing strip coating comprising a two- or three-component system containing a silicone emulsion base, a composition based on a polyurethane dispersion which extends the life of the bath and a crosslinker composition based on a silicone-like or non-silicone-like organic epoxy resin which preferably also contains an organic solvent.
DE-A-19520278 describes one- or two-component urethane paints for coating elastomers containing polyurethane prepolymers with on average two or more isocyanate groups per molecule which are prepared by mixing alcohols having a functionality of two or higher with an excess of isocyanate compounds having a functionality of two or higher and solvent and which contain an extremely large quantity of a compound containing iron oxide for improving surface slip.
EP-A-0375923 describes moisture-curing one-component polyurethane paints for coating elastomers containing polyurethane prepolymers with on average two or more isocyanate groups per molecule which are prepared by mixing alcohols having a functionality of two or higher with an excess of isocyanate compounds having a functionality of two or higher and solvents, reactive polysiloxanes and reactive surfactants being used for improving surface slip. The surfactants are compounds containing a perfluorinated alkyl group with 6 to 12 carbon atoms and a non-fluorinated hydrophilic group with a group reactive to isocyanate-terminated paint ingredients. The paints in question may preferably contain other auxiliaries such as, for example, retarders, accelerators, stabilizers and pigments. However, they may only be applied as very thin films.
The problem addressed by the present invention was to provide one-component water-based coating compositions for coating elastomers which would enable the elastomer profile to be satisfactorily coated and, at the same time, would establish a very low coefficient of friction. In addition, the coatings would show improved abrasion resistance and media resistance in relation to the prior art.
The solution to this problem is defined in the claims and consists essentially in the provision of a suitable polyurethane paint composition containing an effective agent for improving surface slip behavior.
Accordingly, the present invention relates to water-based one-component polyurethane paint compositions for coating elastomers which contain at least one aqueous polyurethane dispersion, a compound containing blocked isocyanate groups, an agent for improving surface slip (friction coefficient), at least one wetting and flow control agent and optionally typical coupling agents, dyes and/or pigments.
The present invention also relates to a process for improving the surface slip of elastomer surfaces, characterized by the following key steps:
optionally cleaning the elastomer surface,
optionally applying a layer of coupling agent to the elastomer surface,
coating the elastomer surface with a composition containing at least one aqueous polyurethane dispersion, a compound containing blocked isocyanate groups, an agent for improving surface slip, at least one wetting and flow control agent and other auxiliaries and additives, coating being carried out by dipping the elastomer profiles in or spraying them with the composition,
drying the elastomer profile for 0.5 to 15 minutes at temperatures of 50xc2x0 C. to 200xc2x0 C. and preferably at temperatures of 90xc2x0 C. to 180xc2x0 C., the binder system crosslinking by chemical reaction after evaporation of the volatile constituents.
The present invention also relates to the use of elastomer profiles thus coated for sealing window channels for moving glass parts and/or as sealing profiles in car assembly, for example as sealing profiles for doors, trunk lids, sliding roofs or engine hoods.
In principle, any aliphatic dispersions based on polyesters or polycarbonates and aliphatic polyisocyanates may be used as the polyurethane dispersions. The polyurethane dispersions contain binders which, essentially, no longer contain any free isocyanate groups although they may contain self-crosslinking groups. It is particularly preferred to use combinations of polyurethane dispersions based on aliphatic polyisocyanates and polycarbonates and on aliphatic polyisocyanates and polyesters. These polyurethane binders are known and standard commercial products and are preferably used for water-based paints.
Any preferably aliphatic polyisocyanate derivatives known per se containing blocked isocyanate groups may be used as the compound containing blocked isocyanate groups.
Actual examples of these blocked polyisocyanate derivatives are reaction products of low molecular weight diols and/or triols such as, for example, ethylene glycol, propylene glycol, butanediol, trimethylol propane, glycerol and the like with a stoichiometric excess of diisocyanates, such as 4,4xe2x80x2-dicyclohexylmethane diisocyanate (H12MDI), 1-isocyanatomethyl-3-isocyanato-1,5,5-trimethyl cyclohexane (isophorone diisocyanate, IPDI), cyclohexane-1,4-diisocyanate, hydrogenated xylylene diisocyanate (H6XDI), 1-methyl-2,4-diisocyanatocyclohexane, m- or p-tetramethyl xylene diisocyanate (m-TMXDI, p-TMXDI), dimer fatty acid diisocyanate, tetramethoxybutane-1,4-diisocyanate, butane-1,4-diisocyanate, hexane-1,6-diisocyanate (HDI), 1,6-diisocyanato-2,2,4-trimethylhexane, 1,6-diisocyanato-2,4,4-trimethyl hexane, butane-1,4-diisocyanate and 1,12-dodecane diisocyanate (C12DI) with subsequent blocking of the free isocyanate groups. Another possibility is to use biuretization or isocyanuratization products of the above-mentioned diisocyanates followed by blocking of the free isocyanate groups.
The blocking agents to be used are selected from aldoximes, ketoximes, lactams, imidazole compounds, xcex2-dicarbonyl compounds such as, for example, malonic acid dialkyl esters or acetoacetic esters. Other blocking agents known per se include alcohols, phenols, more particularly alkylphenols, thioalcohols, thiophenols, secondary amines, amides, imides or hydroxamic acid esters. These blocking groups are then released during curing or crosslinking of the binder system, optionally using catalysts, so that the isocyanate groups released are able to react with other reactive groups such as, for example, hydroxyl groups, epoxy groups, amino groups or carboxyl groups from other constituents of the binder and thus contribute to an increase in molecular weight or to crosslinking of the binder system.
The agent for improving the surface slip behavior of the polyurethane paint composition according to the invention may be selected from powder-form fluoropolymers, more particularly polytetrafluoroethylene (PTFE), and copolymers of perfluoroethylene with other olefinically unsaturated comonomers which may, but need not, contain fluorine, powder-form dialkyl siloxane polymers and/or copolymers or mixtures thereof. Preferred powder-form polydialkyl siloxanes are polydimethyl siloxane powders. Emulsions of an aminofunctional polydimethyl siloxane, an epoxyfunctional polydimethyl siloxane or a hydroxyalkylfunctional polydimethylsiloxane may also be used, optionally in the form of a mixture of various emulsions and/or together with the above-mentioned powder-form fluoropolymers or siloxane polymers. Where emulsions of a polydimethylsiloxane are used, it is important that they contain at least one organofunctional group per molecule which is capable of chemically reacting with the reactive groups of the binderxe2x80x94more particularly with the blocked isocyanate groupsxe2x80x94during the drying step or the curing process so that the polydimethyl siloxane chain is chemically fixed in the binder system and unable to migrate.
Wetting and flow control agents in the context of the invention are any important additives typically encountered in paint technology, including wetting agents and dispersants, optionally thickeners, defoamers, optionally biocides, film forming aids and lubricants. Suitable wetting agents and dispersants are the usual low molecular weight or even polymeric dispersants, for example anionic surfactants, nonionic surfactants or optionally even cationic surfactants. Polyether-modified low molecular weight polydimethyl siloxanes may also be used as wetting aids. Defoamers are known to collect at the surface of the liquid paint or paint film in the form of extremely thin layers and suppress the formation of foam bubbles or cause existing foam to collapse. Effective defoamers are products based on natural or synthetic mineral oils, higher alcohols, modified silicone oils or even polyesters. The flow control agents are intended to promote surface smoothness of the paint films and may be selected from high-boiling solvents used in small quantities and from silicone oils partly modified with polyesters and similar paint auxiliaries. The total quantity of wetting and flow control agents in the formulations according to the invention is up to about 5% by weight, based on the composition as a whole, the sum total of the various wetting and flow control agents preferably being about 3 to 4% by weight.
The polyurethane paint compositions according to the invention may additionally contain typical coupling agents, preferably based on organofunctional silanes, for example aminoalkyl alkoxysilanes, 3-glycidyloxypropyl trialkoxysilane or even vinyl trialkoxysilane. The alkoxy groups are generally C1-4 alkoxy groups.
The polyurethane paint compositions according to the invention may also contain soluble dyes, pigments, flatting agents and other typical auxiliaries. Preferred flatting agents are those based on silica. A particularly preferred black pigment is carbon black.
Since the binder system of the polyurethane paint according to the invention is based on aqueous dispersions, the production process is very simple. The polyurethane dispersions, the blocked polyisocyanate compound, the wetting and flow control agents and the other auxiliaries and the water are mixed together by intensive stirring and the pigments and the agent(s) for improving surface slip behavior are subsequently stirred in. The carbon black pigment may be introduced as a predispersed paste or dispersion of carbon black. The agents for improving surface slip behavior should have a particle diameter of less than 10 xcexcm and preferably of 2 to 5 xcexcm so that there is no need for further grinding of the powder-form constituents with the binder system.
The present invention also relates to the use of the above-mentioned polyurethane paints for the coating of elastomers, more particularly nonpolar elastomers, such as styrene/butadiene rubber, or elastomers based on ethylene/propylene/diene polymers (EPDM) and other flexible substrates, more particularly polyurethanes. The elastomers thus coated show low friction towards glass, particularly in sheet form. This facilitates the movement of displaceable glass panels in vehicles and, compared with uncoated elastomers, also reduces the squeaking noises during driving which can occur with uncoated frames through small sliding movements between the (uncoated) elastomer and other parts of the vehicle, such as the screens or the painted surface of the frame.
In a preferred process for the production of these coated elastomers, the polyurethane paint composition is applied to the still hot vulcanized elastomer profile immediately after leaving the oven either by spraying or by brushing on, the dispersant evaporating almost completely so that there is no need in this case for a separate drying step. The subsequent crosslinking of the polyurethane paint on the elastomer can be carried out by heating for 0.5 to 15 minutes to temperatures of 50xc2x0 C. to 200xc2x0 C. and preferably to temperatures of 90xc2x0 C. to 180xc2x0 C. The dry film thickness of the polyurethane paint is about 10 to 15 xcexcm.